How do you deal with it? Is there anyone else out there whose whole life seemed destroyed after a breakup and you honestly cannot do anything or think of anything else?

Also, if someone has come back to you multiple times, do you think it is likely that they will come back again even if they started dating someone new? If so do you think a relationship can ever work out with someone that keeps leaving?

I can't say I've really had that kind of experience, but I did leave my current girlfriend for another for what I thought were good reasons and then returned to her when I realized that I couldn't really be with someone else.

Many people would probably say that someone who leaves and comes back multiple times has no sense of commitment, is selfish, won't ever work for you, etc, but none of these is necessarily the case. The information you've provided is insufficient to determine the likelihood of either of the two issues you raise. I'm something of a testament to the possibility, but there is no single answer for every situation. If this is personal, you should do your best to analyze the actions of and even confront both the one who continues to leave and the one who accepts him back again and again.

Even if you told us more, we couldn't figure things out for you with any degree of certainty, so I'm afraid this falls on your shoulders. Such is life.